An image forming apparatus using an electrophotographic method includes a fixing device for fixing on a print sheet a toner image transferred to the print sheet. The fixing device includes a heating roller (first rotary body), a fixing roller (second rotary body), a fixing belt extended on and between each of the rollers, a pressure roller, and a heating unit for heating the heating roller to indirectly heat the fixing belt. When a print sheet is conveyed to a nip portion formed between the fixing roller and the pressure roller, a toner image on the print sheet is pressed by the fixing roller and the pressure roller, and additionally heated by the fixing belt. As a result, the toner image is fixed onto the print sheet.
In this type of fixing devices, sometimes the fixing belt slips with respect to the fixing roller etc., and temporarily stops, resulting in a part of the fixing belt in contact with the heating roller to reach an extremely high temperature. As a measure against such a problem, a heating unit of Patent Literature 1 measures surface temperatures of a fixing roller before and after a predetermined time period using a temperature sensor, calculates a slope of temperature difference (temperature gradient) from the measured two temperatures, and determines the fixing belt to be in an abnormal state when the change ratio is larger than a predetermined value.